H2—O2(air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several types of H2—O2 fuel cells including acid-type, alkaline-type, molten-carbonate-type, and solid-oxide-type. So called PEM (proton exchange membrane) fuel cells, a.k.a. SPE (solid polymer electrolyte) fuel cells, are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O2), or air (i.e., O2 mixed with N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin.
During the conversion of the anode reactant and cathode reactant to electrical energy, the fuel cell, regardless of the type, produces anode and cathode effluents that are exhausted from the fuel cell. The anode effluent typically contains unused hydrogen that may be at a concentration that prohibits venting the anode effluent to the environment. The cathode effluent typically contains excess oxygen or air that was not consumed during the electricity production in the fuel cell. The amounts of hydrogen and oxygen remaining in the anode and cathode effluents is dependent upon a number of factors and will vary. For example, the efficiency of the fuel cell can impact the amount of hydrogen and oxygen that are exhausted in the respective anode and cathode effluents. Additionally, the stoichiometry of the fuel cell stack (i.e., the amounts of hydrogen and oxygen that are included in the respective anode and cathode reactants) will also effect the amount of remaining hydrogen and oxygen in the respective anode and cathode effluents.
One of the methods to operate a fuel cell stack is with the anode side of the fuel cell stack deadheaded. That is, the anode effluent produced in the anode side of the fuel cell stack is not allowed to leave the fuel cell stack on a continuous basis. Rather, the anode reactant is supplied to the anode side of the fuel cell stack and remains, under pressure, in the fuel cell stack to consume the majority of the hydrogen during the production of electricity. However, when nitrogen is also present on the anode side of the fuel cell stack, the nitrogen can accumulate to an amount that diminishes the performance of the fuel cell stack. This is caused by the nitrogen preventing the hydrogen from getting to the membrane and inhibiting and/or preventing the production of electricity. Thus, the nitrogen can act as a barrier between the hydrogen and the membrane. Therefore, the anode side of the fuel cell stack is purged from time to time in what is known as a “burping” operation. The purging is done by opening the anode side and allowing the anode effluent to flow out of the fuel cell stack while new fuel, under pressure, is supplied to the anode side inlet. The purging of the anode effluent from the fuel cell stack pulls the nitrogen with it and allows the membrane to become substantially free from the nitrogen and be replenished with hydrogen so that efficient electrical energy production can commence again.
During this purging operation, however, significant amounts of hydrogen can be released in the anode effluent such that the anode-effluent has a hydrogen content that prohibits the venting of the anode effluent to the environment. Therefore, the hydrogen content in the anode effluent must be reduced prior to venting the anode effluent to the environment. Another consideration in the purging operation is that when the anode side of the fuel cell stack is purged, significant pressure differential between the anode and cathode sides can occur. If the pressure differential is of a sufficient magnitude, the membrane separating the anode and cathode sides of the fuel cell stack can be damaged. Additionally, repeated exposure to a pressure differential of a sufficient magnitude can also cause fatigue in the membrane which can lead to premature failure of the fuel cell stack. It is desirable to take these concerns into account when designing and operating a fuel cell system.
Because the hydrogen in the anode effluent may be of a concentration that prohibits venting to the environment directly, typical fuel cell systems employed a tail gas combustor to reduce the hydrogen content in the anode effluent to a level that allows venting to the environment. The tail gas combustor converts the hydrogen into heat that can be used in other parts of the fuel cell system. However, the heat generated by the combustor may only be needed during certain aspects of operating the fuel cell system, such as at startup, and thereafter the tail gas combustor becomes a source of heat that must be dissipated from the fuel cell system. The use of a tail gas combustor also requires complex controls and/or control schemes. The use of a tail gas combustor can also cause pressure differentials when the anode effluent is exhausted to the tail gas combustor such that rupture and/or fatigue of the membrane that separates the anode and cathode sides of the fuel cell stack can occur. These above considerations increase the complexity of a fuel cell system incorporating a tail gas combustor.
Thus, it is desirable to provide a fuel cell system that reduces the hydrogen content in an anode effluent without the use of a tail gas combustor and without subjecting the fuel cell stack to pressure differentials that result in premature failure of the fuel cell stack. Furthermore, it is desirable to accomplish this with a minimal complexity and need for additional controls.